In general, a reluctance machine is an electric machine in which torque is produced by the tendency of a movable part to move into a position where the inductance of an energized phase winding is maximized. In one type of reluctance machine the energization of the phase windings occurs at a controlled frequency. These machines are generally referred to as synchronous reluctance machines. In another type of reluctance machine, circuitry is provided for detecting the position of the movable part (generally referred to as a "rotor") and energizing the phase windings as a function of the rotor's position. These types of machines are generally known as switched reluctance machines. The present invention is applicable to both synchronous and switched reluctance machines.
The general theory of the design and operation of reluctance machines in general, and switched reluctance machines in particular, is known in the art and is discussed, for example, in Stephenson and Blake, "The Characteristics, Design and Applications of Switched Reluctance Motors and Drives", Presented at the PCIM '93 Conference and Exhibition at Nuremberg, Germany, June 21-24, 1993.
As explained above, the basic mechanism for torque production in a traditional reluctance motor is the tendency of the rotor to move into a position to increase the inductance of the energized phase winding. In general, the magnitude of the torque produced by this mechanism corresponds to the magnitude of the current in the energized phase winding such that the motor torque is heavily dependent on the phase current waveforms. For an ideal traditional reluctance motor with no magnetic saturation, the instantaneous torque T, per phase, is: ##EQU1## Where i is the instantaneous current in the energized phase winding and dL/.sub.d.theta. is the derivative of the phase inductance L with respect to the rotor position .theta.. While all practical reluctance motors have some magnetic saturation, this equation is useful for purposes of the present analysis.
As a switched reluctance motor (or generator) operates, magnetic flux is continuously increasing and decreasing in different parts of the machine. The changing flux results in fluctuating magnetic forces being applied to the ferromagnetic parts of the machine and in rapidly varying and pulsating radial forces. These forces can produce unwanted vibration and noise. One major mechanism by which these forces can create noise is the ovalizing of the stator caused by forces normal to the air-gap. Generally, as the magnetic flux increases along a given diameter of the stator, the stator is pulled into an oval shape by the magnetic forces. As the magnetic flux decreases, the stator springs back to its undistorted shape with possible overshoots. This ovalizing and springing back of the stator can cause unwanted vibration and consequently produce audible noise.
In addition to the distortions of the stator by the ovalizing magnetic forces, unwanted vibration and acoustic noise may also be produced by abrupt changes in the magnetic forces in the motor. These abrupt changes in the gradient of the magnetic flux (i.e., the rate of change of the flux with time) may be referred to as "hammer blows" because the effect on the stator is similar to that of a hammer strike. Just as a hammer strike may cause the stator to vibrate at one or more natural frequencies (determined by the mass and elasticity of the stator) the abrupt application or removal of magnetic force can cause the stator to vibrate at one or more of its natural frequencies. In general, the lowest (or fundamental) natural frequency dominates the vibration, although higher harmonics may be emphasized by repeated excitation at the appropriate frequency. These abrupt changes in machine flux often occur at the current commutation instant when the energization of an active phase winding is switched off. This current commutation contribution to unwanted noise and vibration is a difficult problem since such current commutation is an inherent characteristic of conventional switched reluctance machines. Moreover, the problem of unwanted noise and vibration is particulate significant at low rotational speeds where the stator has more time to "spring back" in response to changes in the magnetic characteristics of the machine.
Although the problem of unwanted acoustic noise and vibration has been recognized, known solutions often do not adequately solve the problem and/or require complex, and potentially expensive, controls for controlling the current in the windings of a reluctance machine.
As explained briefly above, in most switched reluctance machines, current in an active phase winding is switched on to produce positive torque when the inductance of the phase winding is increasing and switched off to avoid negative torque when the inductance of the phase winding is decreasing. To produce the appropriate amount of output torque it is important that the magnitude of the current be at a sufficient value over an appropriate portion of the positive-rising inductance region. The inductance of the phase winding, however, limits the rate at which the current in the phase winding may change and tends to slow down the current rise and fall time. This inductance, thus, tends to limit the current available for torque production. Moreover, the back-EMF produced by an operating machine also tends to limit the rate of change of the current and thus potentially limit the torque output of the machine. The limiting effects of the phase winding inductance and the back-EMF become more serious as the rotational speed of the machine increases and the back-EMF becomes greater and the time allowed for current rise and fall time becomes more limited.
A further limitation of conventional reluctance machines concerns the rating of the power converter and DC bus (or DC link) capacitor often required with such machines. It is known in the art that during each stroke period of an active phase winding (e.g., the period over which the phase winding is energized and then de-energized) a significant portion of the electrical energy that is applied to the phase winding when the winding was energized and that is not converted into output torque or motor losses is returned to the power converter. This significant energy circulation characteristic often necessitates the use of power converters having relatively high volt-amp ratings (to provide all of the required power) and in the use of relatively large DC bus (DC link) capacitors (to absorb the power returned to the power converter). Such high rated power converters and large capacitors may significantly add to the overall cost of a reluctance machine system.
It is an object of the present invention to overcome these and other limitations of traditional reluctance machines by, inter alai, providing an improved reluctance machine that has one or more auxiliary damping windings that tend to reduce unwanted noise and vibration in a cost-efficient manner; enhance the current commutation characteristics of the machine so as to provide increased output torque and/or motor efficiencies; and allow for the use of reluctance systems having lower rated power converters and smaller DC link capacitors that would normally be required. Other objects and advantages of the present invention will be apparent to one of ordinary skill in the art having the benefit of this disclosure.